1. Field of the Invention
The present invention relates to a method and an apparatus for superimposing an additional information to a video signal in such a manner as giving least influence on the reproduced image.
2. Description of Related Art
A digital information recording apparatus such as a digital VTR or MD (Mini-Disc) recording and reproducing apparatus are widely spreading and moreover a DVD (digital video disc or digital versatile disc) apparatus providing the recording function has also be developed. In these digital information recording apparatuses, various additional information signals can be recorded in relation to a digital video signal and a digital audio signal as the main information signals and moreover to a computer data.
In this case, this additional information signal is a digital signal and is added to a digital information signal as the signal recorded to the area which may be regionally discriminated from the digital information signal such as a header section which is added, for example, to the data in unit of block of the digital information signal and the other TOC (Table of Contents) area, etc.
In the case of the system where the additional information is superimposed to the existing main information signal for the purpose of recording and transmission, the additional information signal is not superimposed in direct to the digital information signal in the header section but is added to the indirect area. Therefore, the additional information signal can easily be removed by the filtering or alteration, making it impossible, in some cases, to detect the necessary additional information signal in a recording apparatus or a reproducing apparatus. Particularly, when a control information and a copy right information to prevent unfair duplication is added as the additional information signal, it is liable to occur that the predetermined target cannot be attained because such additional information signal is removed.
Moreover, when the additional information signal is added to the indirect area as explained above, if the digital information signal is converted to the analog signal, only the main information signal can be obtained, while the additional information signal is removed. It means that if a duplication preventing control signal as explained above is superimposed as the additional information signal to prevent unfair duplication of the digital information signal, when such digital signal is converted to the analog signal, such duplication preventing means is already lost.
As the additional information signal superimposing system to solve the problems occurring when the additional information signal is removed and when the digital signal is converted to the analog signal, the applicant of the present invention has already proposed a system in which the additional information signal such as a duplication preventing control signal is spectrum spread and the spectrum spread additional information signal is superimposed on a video signal and the video signal is recorded by the digital data or analog data (U.S. patent application Ser. No. 08/75510).
In this system, a PN (Pseudorandom Noise) series code (hereinafter referred to as PN code), for example, which is used as the spreading code is generated in a higher period and it is then multiplied with an additional information signal for the purpose of spectrum spread. Thereby, the additional information signal such as narrow band and high level duplication preventing control signal can be converted to a broad band and very low level signal which does not give any influence on the video signal. This spectrum spread additional information signal, namely the spectrum spread signal is superimposed to an analog signal for the purpose of recording on a recording medium. In this case, the video signal to be recorded on the recording medium may be an analog signal or a digital signal as desired.
In this system of the related art, since the additional information signal such as the duplication preventing control signal is superimposed in the same time and same frequency as the video signal, it is difficult for a person who is trying to conduct illegal duplication to remove the superimposed duplication preventing control signal from the video signal. Meanwhile, it is possible to detect for utilization of the superimposed additional information signal such as duplication preventing control signal by the inverse spectrum spread.
Since the additional information superimposed to the video signal is not removed from the video signal at the time of reproducing operation, the additional information must be superimposed, as is explained above, to the video signal in such a low level as not giving influence on the reproduced image of the video signal. However, although it must be in a low level, the additional information is requested to be in the level which can be detected.
As explained above, the duplication preventing control signal can surely be provided for the recording apparatus side together with the video signal and this duplication preventing control signal can be detected in the recording apparatus side to realize reliable duplication control depending on the detected duplication preventing control signal.
On the other hand, when the video signal is recorded as the digital signal, the video signal is blocked in unit of a plurality of pixels and this video signal is compressed in unit of block. Therefore, it can be thought that one chip of the spread code to produce the spectrum spread signal is assigned to one block and the spectrum spread additional information is superimposed to the video signal.
However, in this case, when the superimposing level per chip assigned to one block is set to the constant level in accordance with the levels xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d of the spectrum spread signal, the superimposing level of the additional information changes in unit of block, resulting in the possibility that the partitioning area of the images in unit of block can be observed.
It is therefore an object of the present invention to provide a superimposing method and apparatus for preventing deterioration of reproduced image due to the superimposed additional information.
The method and apparatus for superimposing the additional information to a video signal of the present invention is characterized in producing, in synchronization with the video signal, a spectrum spread code having a chip section corresponding to each period of a plurality of predetermined pixels of the image of the video signal, producing a spectrum spread additional information signal by spectrum spreading the additional information signal with the spectrum spread code, producing a level changed spectrum spread additional information signal by sequentially changing the level of the spectrum spread additional information signal with the predetermined pattern in the period corresponding to each chip section and then producing an output video signal by superimposing the level changed spectrum spread additional information signal to the video signal.
Thereby, since the level of the spectrum spread additional information signal is not constant in unit of the period corresponding to a plurality of pixels and sequentially changes, the additional information signal can be superimposed to the video signal without observation of the partitioning area in every plural pixels on the reproduced image.